Thermal prestressing of casing

ABSTRACT

A method of prestressing casing in a well to be used in thermal, mineral recovery operations. Following the drilling of the wellbore, casing is lowered into the hole and cement is displaced into the wellbore-casing annulus. Before the cement hardens, the casing is heated to cause elongation. Heating of the casing is continued until the cement hardens. The hardened cement will hold the casing in a state of tension after the well cools. This prestressing method can permit the use of higher well temperatures during thermal operations, can reduce casing cost, and can reduce the probability of well failure.

[73] Assignee United States Patent [54] THERMAL PRESTRESSING 0F CASING 11 Claims, 2 Drawlng Figs.

52 U.S.Cl. 166/288 511 lnt.Cl "012211133413 50 FleldolSearch 166/287,

[ 56] References Cited UNITED STATES PATENTS 1,842,107 1/1932 Lytle 166/291 2,080,406 5/1937 Allen 166/287 2,217,708 10/1940 Scaramucci 166/288 z: mx V121 wow/w; 7/ l 3,189,089 6/1965 Carter 166/288 3,277,963 10/1966 Flickinger 166/291 FOREIGN PATENTS 87,598 1922 Austria 166/288 Primary Examiner-James A. Leppink Attorneys-Jarnes A. Reilly, John B. Davidson, Lewis H.

Eatherton, James E. Gilchrist, Robert L. Graham and James E. Reed ABSTRACT: A method of prestressing casing in a well to be used in thermal, mineral recovery operations. Following the drilling of the wellbore, casing is lowered into the hole and cement is displaced into the wellbore-casing annulus. Before the cement hardens, the casing is heated to cause elongation. Heating of the casing is continued until the cement hardens. The hardened cement will hold the casing in a state of tension after the well cools. This prestressing method can permit the use of higher well temperatures during thermal operations, can reduce casing cost, and can reduce the probability of well failure.

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THERMAL PRES'IRESSING OF CASING BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for cementing well casing. More specifically, the invention relates to a method of thermally prestressing the casing during such cementing operations.

2. Description of the Prior Art The petroleum industry has for many years recognized the desirability of heating oil-bearing formations to increase oil recovery. These thermal operations include such processes as hot-water flooding, in situ combination, and steam flooding and stimulation. While this invention is applicable to any such process where there are periodic variations in the temperature of the well, for convenience it will be discussed with reference to steam injection operations.

One such operation is a process of injecting steam into the well and into the reservoir. The steam drives oil before it to a second, producing well. In another method, a single well is used for both steam injection and production of the oil. The stem is injected through the well and into the formation. Injection is then interrupted and the well is shut in for a heat-soak period. Following the heat-soak, the well is placed on a production cycle and the heated fluids are withdrawn through the well to the surface. Steam injection can increase oil production through a number of mechanisms. The viscosity of most oils is strongly dependent on temperature. With a highly viscous oil, the viscosity may be reduced by 100 fold if the temperature of the oil is increased several hundred degrees. Steam injection can add substantial benefits in recovering even relatively light, low viscosity oil. This is particularly true where such oils exist in thick, low permeability sands where present fracturing techniques are not effective. In such cases, a minor reduction in the viscosity of the reservoir oil can sharply increase productivity. Steam injection is also useful in removing wellbore damage at injection and producing wells. Such damage is often attributable to asphaltic or paraffinic components of the crude oil which clog the pore spaces of the reservoir sand in the immediate vicinity of the well. Steam injection can be used to remove these deposits from the wellbore.

Injection of high-temperature steam which may be 600 F. or even higher does, however, present some special operational problems. When steam is injected through the tubing, there may be a substantial transfer of heat across the annular space to the well casing. When the well casing is firmly cemented into the wellbore, as it often is, the thermally induced stresses may result in casing failure. As the casing temperature rises, the casing attempts to elongate. If it is firmly bonded by the cement, the casing will be unable to expand and severe compressive stresses may result. The casing may in fact fail. Even where compressive failure does not occur, there may be severe deformation of the casing during the periods while it is heating. Such deformation can cause weakening of the casing, particularly at the threads between the casing joints. When the well cools, the deformed casing contracts. During this contraction, the deformed and weakened casing often fails.

A number of proposals have been advanced to combat this problem. It has been suggested, for instance, that extremely high strength casing be employed in a thermal well. This technique reduces the probability of easing failure but, quite naturally, increases the cost of the well.

Another technique which has been suggested for combating the problem of thermally induced stresses in casing is mechanical prestressing. In this technique, only the bottom portion of the casing is cemented or tacked in place. After this initial cementing is completed the casing is placed in a state of tension by imparting a strain to the string. This strain is generally imposed by using casing jacks at the surface to stretch the string. While the casing is in a state of tension a second stage of cement is introduced into the uncemented portion of the wellbore-casing annulus. The mechanical tension is maintained on the casing until the cement has hardened. One obvious disadvantage of this method is that it requires at least two stages of cementing and an attendant loss of rig time. Moreover, there is a possibility that as the tensile stress is imposed on the casing, the cement at the bottom of the hole may fail or the casing string itself may part.

Another suggested technique is the bitumastic completion. In this technique, the casing is first coated with a material which will melt at high temperature and then is cemented in the wellbore. When the well is heated, the casing coating melts and the casing is free to expand or contract within the wellbore without excessive stresses being imposed on the casing. A serious difficulty with this technique is the vertical displacement of the wellhead as this casing expands and contracts. The wellhead equipment may move several feet in a typical cycle. This creates obvious operational problems with the surface equipment at the wellhead.

SUMMARY OF THE INVENTION This invention relates to a method for thermally prestressing well casing. The casing is heated and allowed to expand before the cement has hardened and set. After the cement has hardened and the well cools, the casing is in a state of tension. During subsequent thermal operations with the well, this tensile stress must be relieved by heating of the casing before any compressive stresses can be generated. Such prestressing permits the use of higher well temperatures, the use of lower grades of casing, and reduces the probability of well failure.

A primary object of this invention is to provide a method for prestressing casing to be used in a thermal well operation.

Another object of this invention is to provide a thermal method for prestressing such casing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of a vertical section of the earth showing a well system containing casing and tubing strings.

FIG. 2 is a schematic representation of a vertical section of the earth showing an alternative embodiment of a well system containing casing and tubing strings.

DETAILED DESCRIPTION OF THE INVENTION This invention can perhaps be most readily understood by reference to the drawings. In the embodiment shown in FIG. I, a well shown generally at 10 has been drilled from the surface of the earth 11 to an oil-bearing formation 12. A string of relatively large diameter steel pipe or casing 13 has been lowered into the hole so that it extends from a point near the bottom of the well to the surface of the earth. A float shoe l4 and one or more float collars 15 may form a part of the casing string. These float devices are simply check valves which permit the flow of fluids from the interior of the casing to the easing-wellbore annular space and prevent flow in the opposite direction.

After the casing has been lowered to the desired depth, the cementing operation is begun. Cementing operations are well known oil well completion techniques and need only be generally described herein. In such an operation, the cement is introduced into the casing at the top of the well and pumped downward. The cement slurry' asses through the float devices and then up the annular space 17 toward the surface. The cement slurry is conventionally displaced from the casing by injection of water. A cementing plug 16 is normally employed to separate the cement from the displacing water and to wipe off any excess cement which adheres to the walls of the casing. Thus at the end of this operation as shown in FIG. 1 the wellbore-casing annulus is filled with the cement slurry l8 and the casing string is filled with water. The cementing plug 16 rests over the topmost float device and prevents the downward passage of fluid within the casing.

In the practice of this invention the casing is caused to elongate by heating before the cement has hardened. This heating may be accomplished in any suitable fashion and with any convenient source of thermal energy. However, it is preferred in most instances to accomplish this heating with steam due to its ready availability. ln a large scale steam flooding operation, steam from a central steam generator may be used. Portable steam generators capable of producing 80 percent quality steam at 1500 p.s.i.g. at 5000-l0,000 pounds of steam per hour are commercially available and may be used to supply steam as a heat source.

Mixtures of steam and noncondensing gases such as air or natural gas are suitable sources of heat. Such mixtures can be particularly helpful in removing condensed steam from the wellbore since these mixtures can have higher pressures than steam at the same temperature.

To accomplish this heating in a typical operation, a smalldiameter pipe or tubing string 19 is lowered into the waterfilled casing. At the surface the tubing is attached to a vent line 20 having a control valve 21. High-pressure steam from a portable steam generator (not shown) is introduced into the casing through the inlet line 22 having a control valve 23. The high-pressure steam displaces the water down the casing and up the tubing as is indicated by the arrows.

It should be apparent that it is necessary to remove the water within the casing before the casing can be efficiently heated. Otherwise excessive quantities of heat will be consumed in heating and vaporizing the water. Unless the pressure requirements are too high, this is perhaps best accomplished by displacing the water with steam as previously described. It should also be apparent that there are a number of alternatives for removing the water from the casing. For example the water may be swabbed or pumped through the tubing with a number ofconventional means.

After the water has been displaced from the casing, steam is circulated down the casing and up the tubing as indicated by the arrows. The circulating steam will cause the casing string to elongate by thermal expansion. The amount of this elongation can be readily determined from the length of the casing string, the coefficient of thermal expansion for steel and the change in temperature of the casing due to circulating steam.

The temperature of the circulating steam should be chosen so that the desired elongation of the casing is obtained. If the temperature is too high the casing will elongate by an excessive amount and when the cement hardens and the casing cools, the casing may fail due to excessive tension. The relationship between the design strength of the casing and temperature is given by:

S T, OLE T,

where T, equals the design casing temperature, F.

S equals the casing design strength, p.s.i. (1 equals the coefficient of thermal expansion for steel, ft/ft- E is equal to the longitudinal modulus of elasticity for steel.

T, is equal to average surface temperature, F.

The design strength of casing is conventionally stated in terms of the minimum yield strength and minimum tensile strength of the pipe. The yield strength is the force per square inch that can be imposed on the pipe before plastic deformation will occur. The tensile strength is the force per square inch that can be imposed before the pipe wall will fail in tension. For example APl grade J-55 casing has a minimum yield strength of 55,000 p.s.i. and a minimum tensile strength of 75,000 p.s.i. Thus this grade of easing can stand a minimum axial force of 55,000 pounds per square inch of pipe wall before it deforms plastically. it will stand a minimum force of 75,000 p.s.i. before it will fail in tension.

For convenience, the temperature which will induce a stress in the casing which is equal to its minimum yield strength will be referred to as the yield strength temperature. The temperature corresponding to the tensile strength will be referred to as the tensile strength temperature.

The temperature of the injected steam should always be below the tensile strength temperature. Preferably the temperature should be equal to or less than the yield strength temperature. Moreover in determining the steam temperature other forces which may be imposed on the casing should be taken into consideration. For example the weight of the casing string will impose a tensile stress on the pipe which will reduce the maximum steam temperature. Conversely, the buoyant forces on the casing will increase the maximum temperature.

The proper composition for the cement slurry to be used in the practice of this invention can be readily determined using known and available technology. Cement compositions for use in high-temperature wells have been under investigation for a number of years. Suitable cement compositions for wells having bottom-hole temperatures as high as 700 F. have been described in the literature. Typical disclosures are contained in: Cain et al. Cementing Steam Injection Wells in California, JOURNAL OF PETROLEUM TECHNOLOGY, Apr. 1966; Ostroot et al. Improved Compositions for Cementing Wells with Extreme Temperatures, JOURNAL OF PETROLEUM TECHNOLOGY, Mar. 1961; Walker, Cementing Compositions for Thermal Recovery Wells," JOUR- NAL OF PETROLEUM TECHNOLOGY, Feb. 1962; Ostroot et al. Cementing Geothermal Steam Wells, JOURNAL OF PETROLEUM TECHNOLOGY, Dec. 1964.

While design of the cement composition is primarily a matter of applied engineering technology, the specific requirements of the method of this application should be considered in preparing the cement slurry. A paramount consideration is that the slurry should not setup before the desired casing temperature has been reached. The setting time of cement slurries is strongly dependent upon temperature and, unless the proper precautions are taken, premature setup may occur. Retarding agents may be added to the slurry to prevent such a premature setup. Typical retarders are gypsum, sugar, lime, various gums and sodium tannate. Lignin retarders, such as calcium lignosulfonate, and mixtures of lignin retarders with carboxymethyl hydroxyethyl cellulose have been found to be particularly effective in retarding the setup time of cement slurries. Thickening times at high temperature of many hours can be obtained using such retarders.

High temperatures are detrimental to the strength of ordinary cements. lt has been found that the addition of silica flour to the cement slurry will produce a hardened cement having good strength at high temperature.

Density of the cement slurry is another factor which should be considered. The density of the cement should not be so high that there is a danger of the casing collapsing when the water is withdrawn from the casing prior to steam injection. Finally, it is a prudent practice to test the designed slurry composition before actual use in the field. Conventional laboratory techniques can be used to determine whether a given cement slurry has the desired properties.

FIG. 2 illustrates an alternative embodiment for carrying out the method of this application. A well shown generally at 30 has been drilled from the surface of the earth 31 to the formation of interest 32. A string of casing 33 has been run into the wellbore and a string of tubing 34 has been lowered into the hole inside the casing string. One or more float devices (float shoes and float collars) 35 are attached to the lower end of the tubing string. As was previously stated these devices are simply check valves which will permit the flow of fluids out of the tubing string but will prevent flow in the opposite direction. The lower end of the tubing-casing annulus is sealed by suitable means such as a drillable, thermal packer 36. Just above the packer is a plug-operated sliding sleeve 37 in the tubing. The top of the tubing string is attached to an inlet line 38 with valve means 39. A vent line 40 with valve means 41 is attached at the casing head.

In this embodiment of the method, a cement slurry is introduced through the tubing inlet line 38, displaced down the tubing and out the float device 35. After the desired quantity of cement has been injected into the tubing, a cementing plug 42 is inserted in the flow line at the surface. The cementing plug and the cement ahead of it are then displaced down the tubing by high-pressure water. Displacement of the cement from the tubing causes the cement to rise in the wellbore-casing annulus to the surface. When the cementing plug 42 reaches the plug-operated sliding sleeve 37, it forces the sleeve downward and exposes ports in the sleeve assembly at that point. Internal projections on the sliding sleeve prevent the plug from moving below that position and the plug blocks the flow of fluid below its position. Thus after the cementing plug has opened the sleeve, the cement slurry is in a proper position around the exterior of the casing and a path for fluid flow exists from the tubing inlet line 38, across the ports of the sliding sleeve 37 and out the outlet line 40. The water within the tubing and the casing is then removed in one of the ways previously described. High-temperature, high-pressure steam is then introduced into the tubing and displaced up the tubing casing annulus. The steam injection is continued until the cement has hardened. After the cement has hardened and the well has cooled, the casing will be prestressed in a state of tension.

The following example will further illustrate this invention:

EXAMPLE A well is drilled with a 9 inch bit to penetrate the formation of interest which lies at a depth of 1450-1485 feet from the surface. After drilling the well to a total depth of I525 feet, the casing string is lowered into the hole. The casing string has a total length of 1495 and is made up of AH grade J- -55 pipe with a 7 inch OD. The casing weighs 26 pounds per foot and is provided with standard buttress thread couplings. The casing string is equipped with a float shoe at the bottom and a float collar at the top of the first joint above the bottom. A cement composition of AP] class neat cement, 35 percent silica flour and a lignincarboxymethyl hydroxyethyl cellulose retarder mixture is mixed with water at the surface and injected down the casing. After a quantity of cement slurry which is sufficient to fill the annular space between the wellbore and the casing has been injected, a cementing plug is inserted in the casing at the cementing head. Water is then pumped into the casing to displace the cement slug and slurry. Water injection is continued until a sharp pressure rise at the surface indicates that the plug has reached the float collar. Steam injection tubing is then run into the hole. The tubing has a 3%inch OD and is approximately 1450 feet in length. The displacing water is then removed from the casing and the tubing by swabbing the tubing with a sand line on a drilling rig. The average temperature at the surface of the well is approximately 80 F. and the temperature at the casing float shoe is approximately 110 F. It is determined that the yield strength temperature for this casing is approximately 355 F. The buoyant force on the casing string and the weight of the casing string counterbalance and therefore no adjustment is necessary to correct for these forces. A portable steam generator is then connected to the casing head. The generator is adjusted to produce 80 percent quality steam at 355 F. and 120 p.s.i.g. at a rate of 5000 pounds of steam per hour. Steam injection is continued for 24 hours. At the end of this period the casing string has elongated by approximately 2.7 feet and the cement slurry has hardened. After the well has cooled, the induced tensile stress in the casing is approximately 55,000 p.s.i.

While this invention has been described with reference to a steam injection well, it should be understood that the method of this invention is equally applicable to any well used in thermal operations. The method could be applied in wells which are used to produce the hot fluids in steam drive or in situ combustion operations. It is also contemplated that the method may be used in wells for recovering minerals other than petroleum. For example, subterranean sulfur deposits are often exploited using cased wells and thermal methods. This invention has obvious applicability to such operations.

What I claim is: l. A thermal, mineral-recovery method in which a wellbore containing a pipe extends from the earth's surface to a subterranean mineral-bearing formation which comprises:

a. placing a cement slurry in the annular space between the pipe and the wellbore;

b. then, introducing into the pipe a source of thermal energy to heat the pipe and to cause the pipe to thermally elongate;

c. maintaining the pipe in a heated state until the cement slurry hardens; and

d. subsequently, conducting a heated fluid through the pipe between the earth's surface and the mineral-bearing formation.

2. A method as defined by claim 1 wherein the pipe is heated to a temperature which is less than the tensile strength temperature.

3. A method as defined by claim 1 wherein the pipe is heated to a temperature which is less than the yield strength temperature.

4. A method as defined by claim 1 wherein the cement slurry contains a retarding agent.

5. A method as defined by claim 1 further including displacing water from the casing prior to introducing the source of thermal energy.

6. A method as defined in claim 1 wherein the source of thermal energy is steam.

7. A method as defined in claim 1 wherein the source of thermal energy is steam which is injected down the annular space between the casing and the tubing and withdrawn through the tubing.

8. A method as defined in claim 1 wherein the cement slurry is placed in the annular space by injection down a tubing string having at least one float valve.

9. A method as defined in claim 1 wherein the pipe has at least one float valve.

10. A method as defined by claim 9 wherein the pipe has a plurality of float valves arranged in a series.

11. A method of cementing a casing string in a well to be used in the production of oil from a subterranean formation by thermal methods comprising:

a. introducing a cement slurry into the casing string;

b. displacing the cement slurry from the casing string and into the annular space between the casing string and the well with water;

c. placing a tubing string within the casing string;

d. removing water from the casing string;

e. injecting steam down the casing-tubing annulus and out the tubing string to heat the casing string to a temperature which is approximately equal to the yield strength temperature of the casing; and

f. continuing to heat the casing string with the steam until the cement slurry hardens. 

2. A method as defined by claim 1 wherein the pipe is heated to a temperature which is less than the tensile strength temperature.
 3. A method as defined by claim 1 wherein the pipe is heated to a temperature which is less than the yield strength temperature.
 4. A method as defined by claim 1 wherein the cement slurry contains a retarding agent.
 5. A method as defined by claim 1 further including displacing water from the casing prior to introducing the source of thermal energy.
 6. A method as defined in claim 1 wherein the source of thermal energy is steam.
 7. A method as defined in claim 1 wherein the source of thermal energy is steam which is injected down the annular space between the casing and the tubing and withdrawn through the tubing.
 8. A method as defined in claim 1 wherein the cement slurry is placed in the annular space by injection down a tubing string having at least one float valve.
 9. A method as defined in claim 1 wherein the pipe has at least one float valve.
 10. A method as defined by claim 9 wherein the Pipe has a plurality of float valves arranged in a series.
 11. A method of cementing a casing string in a well to be used in the production of oil from a subterranean formation by thermal methods comprising: a. introducing a cement slurry into the casing string; b. displacing the cement slurry from the casing string and into the annular space between the casing string and the well with water; c. placing a tubing string within the casing string; d. removing water from the casing string; e. injecting steam down the casing-tubing annulus and out the tubing string to heat the casing string to a temperature which is approximately equal to the yield strength temperature of the casing; and f. continuing to heat the casing string with the steam until the cement slurry hardens. 